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The Subtleties of Color

By Sarah Sands

Small Differences That Make All the Difference

Every painter knows the dance, taking a few steps back from the painting,
their head tilted slightly askew, the eyes pulled tight into a squint, or
the hand held in front to block off an area from view. The to and fro of
action and adjustment, of sense and sensibility. And if color happens to be
foremost in that dance, the search is often for still unseen subtleties to
be coaxed from the colors at hand. Even if the effect is one of clash, there
remains the desire for it to be 'just so'; a clash tuned to the highest
pitch or given a particular piquant flavor. In the end, artists thrive on
subtleties, on the small differences that make all the difference, and their
search for colors that can respond to those needs is endless.

All these immeasurable things, no matter how rarefied they might seem,
ultimately have a material basis rooted in the nature of pigments and paint,
the common tools of the trade. In the pages ahead we will start by reviewing
some of these underlying factors and then examine why some of the most
subtlest of variations can make all the difference in choosing the right
color matching your intent.

Beyond All Measure: the Limitation of Colorimetry

When trying to describe what makes any particular color unique it is
tempting to point to a color's location within a well defined system such as
CIE L*a*b* or Munsell. Doing so allows us to feel that we can place the
color's uniqueness within a mapped and measured space, and even calculate
the degree of difference it has from all those other colors that jostle for
a treasured spot of their own. But we would quickly learn that it is
precisely those subtleties that are lost in the process. For all the
accuracy of our spectrophotometer in reading the exact makeup of the light
being reflected back from a sample, nothing in that information really tells
the artist what they need to know about actually using that paint: how it
mixes with other colors, its degree of opacity, tinting strength, or any
number of physical attributes. Paint is ultimately color on the move,
dynamic and energetic, and no single snapshot can capture that more vibrant
life lived on the palette of the studio.

While clearly no system of measurement is perfect, spectral reflectance
curves perhaps come closest to capturing the nuance of a color, especially
if the spectrum is available for both masstone and a tint of a known
percentage. With these two points as reference one can roughly gauge how a
particular paint might perform in mixtures. However, care must be taken as
even here there are difficulties. Cadmium and Hansa Yellow Medium, for
example, might share nearly identical spectra at full strength but no one
would mistake them for the same in practice. Additionally, one must factor
in the responsiveness of the eye to various wavelengths, as this can greatly
shape how the eye perceives the color -- which can be very different from the
data itself.

Underlying Causes of Subtle Differences

Nearly all of the subtleties of a particular color can be traced back to the
physical attributes of the pigments and the way light interacts with those
particles within a paint film through absorption, reflectance, scattering,
and transmission. Pigments, in turn, are largely characterized by their
underlying chemical composition, along with such factors as particle size,
refractive index, and scattering coefficient, while paint films impact color
through their pigment load, thickness, and sheen. Ahead we will touch on all
of these briefly, as a way to introduce some of the complexity behind many
of the subtle differences we see.

Physical Properties of Pigments

Crystalline Structure of Pigments

All pigments, with rare exception, have crystalline structures that dictate
their color, and even small changes at this level can alter which
wavelengths are absorbed or reflected. Phthalocyanine Blue, for example,
has two types of crystal formations (α and β ) that are
responsible for their slight leanings toward the Red or Green Shade, while
changes to the crystal lattice of Quinacridone is responsible for its broad
range that runs the gamut of bright Quinacridone Reds to the ever deeper
Magentas and Violet. A third example includes the entire array of Cadmium
colors, where cadmium sulfide, which is yellow in its pure state, is made
progressively redder and deeper by replacing the sulfur in the crystal
lattice with increasing amounts of selenium. This substitution broadens the
amount of the spectrum that can be absorbed, and if enough selenium is
added, Cadmium can actually appear black.

Transparency/Opacity and Tint Strength

A particle's opacity is greatly dependent on its ability to scatter light,
which relies primarily on a particle's refractive index and size. The larger
the difference in the refractive index between a particle and its
surrounding medium, the more light is scattered and the underlying layer
obscured; a phenomenon similar to the way fog scatters a car's headlights.
Conversely, the closer these numbers are, the more transparent a particle
will appear. The high refractive index of Cadmium Yellow and Titanium White,
for instance, is almost solely responsible for their tremendous hiding power
and sense of opacity, while Zinc White and Hansa Yellow appear more
transparent because their refractive indexes are considerably closer to that
of an acrylic polymer. Because dark pigments with low refractive indexes,
such as the Phthalocyanines, do not scatter much light, their hiding power
resides almost completely in their ability to absorb light, the pigment
loading, and the thickness of the film.

The other aspect of particle size has equally dramatic consequences on both
scattering and tinting strength. As a particle becomes smaller it scatters
light more effectively until a certain optimal size is reached, after which
this aspect begins to drop off sharply. As one continues further below this
threshold, the pigment particle grows increasingly transparent while
simultaneously reaching a maximum of tinting strength. Here is where the
magic of the Transparent Iron Oxides reside, as the normally opaque iron
oxide pigments are manufactured to such small particle sizes that they
become wonderfully translucent and far more effective in glazing and the
production of cleaner, higher chroma tints. Titanium White, on the other
hand, is carefully manufactured to optimize its particle size for maximum
light scattering, and hence opacity. In fact, a one centimeter wide crystal
of Titanium Dioxide is completely transparent, and it is only as the
crystals get smaller that scattering becomes dominant and we sense the
pigment as inherently 'white'; an effect similar to the whiteness of finely
ground glass. Should the Titanium Dioxide be ground even further down to a
nano-particle scale, it would actually become completely transparent, a feat
that seems almost magical given how strongly we associate opacity with
Titanium White.

Purity and Uniformity

Differences in the chemical purity of a particular pigment, as well as the
uniformity of its shape and size distributions, are responsible for still
other quirks of coloration. For example, natural earths owe their particular
flavors and nuances to varying amounts of trace elements, such as manganese
oxide, silica, alumina and clays, as well as their wide assortment of
particle sizes. While this accounts for many of their prized undertones, and
explains why particular regions in the world become coveted for their mined
ochres, siennas, and umbers, it is also the reason why these colors are
generally weak tinters and lower in chroma than the parallel range of
synthetic oxides. Also, because they are mined, these pigments have a wide
lot to lot color variation depending on the level of impurities in the next
shovel full. Ultramarine Blue presents another example; one of the earliest
synthetic pigments, it is richer and more saturated than the genuine Lapis
Lazuli it replaced, which as a mined rock always came with impurities of
calcite, sodalite, and pyrite, that muted its tone.

The Physical Properties of Paint

Film Thickness

As most artists know, colors do not necessarily stay the same through thick
and thin. In thick films of densely packed pigment, the masstone is dominant
and the color will appear more saturated and deeper. As the film becomes
thinner, the undertone becomes more pronounced and the overall color can
appear more transparent, lighter in value, and sometimes higher in chroma as
well, assuming the underlying substrate is very light in tone. These effects
are ultimately caused by having an increased amount of light reflected from
both the pigments and the underlying substrate in the form of backscattering.

Pigment Load

Beyond film thickness, simply altering the pigment load or density in a
paint film can markedly change the perception of a color. For example, in a
film of densely packed translucent pigments, much of the interior scattering
and transmittance of light can be lost through subsequent and repeated
absorption, and one primarily sees just the reflected light coming from the
surface. This reduction in light reads as a deepening in hue and a
reinforcement of the dominant absorption band. As pigment load is decreased,
and light begins to penetrate through the film, the interplay of scattering
and absorption has a larger impact on the overall color. One can imagine a
similar effect if placing identical sheets of stained glass on top of each
other, one after another. As the pile grows thicker, the color will get
increasingly deeper and more saturated.

At its most extreme, the spectral reflectance curve can change considerably
as more and more light is able to penetrate deeply into the material. This
phenomenon can be seen in such transparent colors as Green Gold and Nickel
Azo Yellow, where a dramatic difference emerges between the mass and
undertone, as well as a subtler shift in spectra for Phthalocyanine Blue
G/S.

Sheen and Surface

Whether a surface is glossy or matte, smooth or textured, will ultimately
impact a color's expression as well. As a paint film becomes glossier and
smoother, there is less scattering of light at the surface and more
penetration and absorption of the light by the pigments themselves. This
causes darker colors to typically appear deeper and more saturated when they
have a gloss sheen, and conversely, appear to lighten if matte; not unlike
the phenomenon of removing a darker colored stone from the bottom of a
riverbed and watching the seemingly rich color dissolve before ones eyes
with the evaporation of the water.

Case Studies

The spectral data used in the following case studies was obtained using a
Minolta® Spectrophotometer. Samples were as 10 mil drawdowns on lacquered
cards, with each color represented both at full strength and mixed with
varying percentages of GOLDEN Regular Gel (Gloss) or Titanium White. Many of
the graphs used in this article are spectral curves, which might be
unfamiliar to many artists as they are not that common outside of laboratory
settings. The easiest way to understand them is simply as showing the amount
of light that is reflected from the surface for each wavelength in the
visible spectrum. The more that is reflected, the higher the curve will be
at that point. To make the readability a little easier, along the top of
each graph we include markings showing the approximate range for each band
of color, running from Violet through Blue, Green, Yellow, Orange, and Red.
The x-axis, running along the bottom, is marked with the actual wavelengths
themselves.

Graph 1

Quinacridone Violet and Quinacridone Magenta Masstones

Family Resemblances

Quinacridone Violet and Quinacridone Magenta Masstones

Quinacridone Magenta and Violet are one of those common cases of colors that
seem so close together surely it couldn't matter all that much which an
artist reaches for. Of course the answer depends somewhat on your needs.
Quinacridone Violet is more opaque and bluer in the undertone than its more
transparent, redder cousin. While these features go easily unnoticed when
used full strength, the subtleties become much more pronounced when tinted
or used in transparent glazes, as can be seen in the spectral graphs. Notice
how the spectral curves of the masstones of Quinacridone Violet and Magenta
(Graph 1) have virtually identical shapes, with almost negligible levels of
reflectance from Violet all the way through Yellow (400-600nm) until finally
rising sharply within the cooler, outlying regions of Red. If one looked at
these two colors, it would be difficult to tell them apart. However, mixing
these colors 1:10 with Titanium White or 1:50 with Regular Gel not only
dramatically changes the shapes of their respective spectra but clearly
highlight their differences as well. In both the transparent let downs
(Graph 2) and the tints (Graph 3) of these colors one can 'see' the warmer
aspect inherent in the Quinacridone Magenta, where its spectra now rises
much earlier, indicating a new more orange component, while the Quinacridone
Violet continues to exhibit strong absorption even past 600nm.

Another aspect to notice is the difference between the mixtures of
Quinacridone Magenta with gel versus Titanium White (Graph 4). While the one
with gel reaches a level of reflectance for cooler reds that is nearly equal
to the same mixtures with white, there is a continued, extremely shallow
level of absorbance in the 525-575nm range, which would be descriptive of a
complementary shade of Green. Because this complement is suppressed, this
transparent mixture is able to possess a very high and brilliant chroma,
creating a scintillating pink that is impossible to achieve when adding
white. It's a good lesson to remember for those constantly frustrated with
an inability to hit that jarringly high note. And the reason is easy to see.
With the addition of white the spectral profile starts to flatten out, with
more and more light in the Green range being reflected, which ultimately
results in a loss of chroma and a 'chalkiness' as the cooler tones begin to
essentially cancel or grey-out their warmer compliments.
Link to additional supporting visuals

Phthalo Blue (GS) (PB 15:3) / Phthalo Blue (RS) (PB 15:1)

These twins present an interesting conundrum where they start off
ever-so-slightly reversed in terms of which masstone has a more measured red
or green cast, with Phthalo Blue (GS) initially having a small edge in the
red zone and an even greater lean towards the warmer, violet end of the Blue
range. As the colors are let down or tinted those positions reverse
themselves and the warmer undertone of Phthalo Blue (RS) finally comes to
the fore. One can see this in the accompanying graph (Graph 5), where the
Phthalo Blue (GS) starts, oddly enough, with actually more red then its
supposedly warmer sibling Phthalo Blue (RS). However, once mixed with white,
the Green Shade finally assumes its rightful place, passing across the
trajectory traced by the Phthalo Blue (RS) and comfortably out-distancing it
along the green axis. This peculiar flip-flop holds true even when
extending these with gel and can be clearly felt when mixing with yellow to
create various greens.
Link to additional supporting visuals

Carbon Black (PBk 7) / Mars Black (PBk 11)

"Black is Black", as the old Los Bravos song goes, although of
course that truism doesn't ring true within the domains of paint. While
their masstone spectra present somber flat lines across the bottom of a
spectral reflectance chart, with scant a sign of difference, things change
after examining mixtures, where the pronounced warm undertone of Mars Black
can quickly becomes noticeable. In the graphs below, both Carbon and Mars
Black start off extremely close in their balancing of the two principle
axis of the CIELAB color space -- namely A (Red/Green) and B (Yellow/Blue.)
However, after mixing each sample 1:50 with Regular Gel, one can see that
the Carbon Black has essentially not budged, maintaining a near neutral
equilibrium. Mars Black, by contrast, quickly reveals a pronounced brownish
undertone, seen here as a trending upwards in the lower Red and Yellow
quadrant.
Link to additional supporting visuals

When Colors Coincide

The Hansa Yellows sit across from the Cadmiums like a row of twins arriving
late and uninvited to a family dinner. Contrary to many notions these are
not the poor substitutes for the 'real' thing, but truly flushed with their
own sense of flash and purpose within the painter's toolbox. The Hansas
might not have the opacity of the Cadmiums, but their transparency allows
them to be an essential ingredient for transparent glazes, deep greens, and
composite blacks. For all the brash and brawn of the Cadmiums, the Hansas
speak in their own bright voice.

A good way to experience these subtleties is to watch how the two colors
impact various mixtures. With a Phthalocyanine Blue or Quinacridone Magenta,
for example, Cadmium Yellow Medium creates dense lighter-valued tints with a
sense that white has somehow strayed into the mix. In the accompanying graph
(Graph 7), notice how it produces a sharp spike in value after 500nm, and an
elevated reflectance throughout the oranges and reds. With Hansa Yellow
Medium, on the other hand, the saturation of Phthalo Blue (GS) is largely
preserved and the hue is simply shifted towards green with a minimum
increase in value. What is not as well shown here is the fact that the
translucency is held onto as well, the mixture remaining ideal for glazing
and developing other rich, dark greens.

Pyrrole Red (PR 254) / Cadmium Red Medium (PR 108)

Like the Hansas, the drama of the Pyrrole Reds and their Cadmium
doppelgangers is often played out through mixtures more than masstones. At
full strength both Cadmium Red Medium and Pyrrole Red are opaque with
spectra that closely echo each other with just the slightest amounts of
difference, causing Pyrrole Red to appear slightly warmer and higher in
chroma. Little there, however, prepares us for the sharper divergence that
occurs in the tints with Titanium White. The Pyrrole and Cadmium are now
equivalent for the majority of red wavelengths, though Pyrrole Red still
maintains a greater reflectance in the oranges and, most significantly,
shows much stronger absorbance throughout the greens and on into the blue
regions before a slight bump up in the violet. As a result, the Pyrrole Red
tint appears richer, deeper and less chalky then the more neutralized
Cadmium Red Medium. As one can imagine, these differences in tint strength
and chroma get repeated in nearly every mixture where these colors play a
major role.

Seeing Through Opaque Pigments

This section begins with pairs of synthetic and natural iron oxides whose
differences revolve almost entirely around particle size, with the synthetic
oxides being exceptionally small when compared to the usually chunky,
larger-scaled pigments of natural earth colors. As mentioned earlier, when
particles grow smaller not only does the total surface area increase
rapidly, but their ability to scatter light diminishes as well. With
scattering held to a minimum, the pigments' interaction with light is solely
through absorption and reflection, which both maximizes their tinting
strength and increases their translucency. As a result, the synthetic oxides
will often be the preferred choice when needing brighter mixtures and
cleaner glazes, while the more standard earths can provide a wonderful
opacity and density when relying on their masstone.

Burnt Sienna (PBr7) / Transparent Red Iron Oxide (PR 101)

In this grouping, well-known Burnt Sienna is contrasted with the similarly
hued Transparent Red Iron Oxide. Both start off as mid-toned earths, the
Burnt Sienna a touch brighter and with a slightly higher reflectance in the
warmer orange to yellow range, while the Transparent Red Iron Oxide reads as
a ruddy and rich mahogany brown, with its peak reflectance deep within the
cooler range of reds. None of that, however, quite prepares one for the
transformations that happen when the samples are tinted with white or mixed
with gel to form a glaze. As the graph shows (Graph 10), for example,
Transparent Red Iron Oxide jumps dramatically in Chroma, or Saturation, even
when mixed as high as 1:1 with Titanium White. By contrast, Burnt Sienna
remains very low in Chroma, never raising much beyond its starting point, as
it forms the tell-tale cold and pasty pastels of nearly every brown when
mixed with white alone. Similarly, when making glazes, the Transparent Red
Iron Oxide blooms into rich browns with bright, fiery undertones of orange
while the Burnt Sienna will always carry a slight sense of murkiness.

Yellow Oxide (PY42) / Transparent Yellow Iron Oxide (PY42)

Yellow Oxide and Transparent Yellow Iron Oxide have differences that are a
little obscured, perhaps, by their identical Color Index designation as
PY42. In fact, many artists assume far too often, that pigments with
matching Color Index names are unvaryingly the same. But nothing could be
further from the truth, especially if reaching for a yellow earth for
glazing or to use in tints or mixtures. And some differences can be seen
fresh out of the tube, where the Yellow Oxide starts out brighter and very
opaque, while the Transparent Yellow Iron Oxide has a much deeper, almost
Raw Sienna masstone and is one of our most transparent colors. From there
the differences grow, all tied to the singular issue of particle size more
than anything in their chemistry.

Yellow Oxide (PY42) / Transparent Yellow Iron Oxide (PY42)

In the first accompanying graphs we show both colors in their masstone and
mixed 1:10 with Regular Gel Gloss, a ratio similar to one often used for
glazing. The Yellow Oxide, shown in the two lighter lines, is true to its
strong opacity and changes very little considering it has been mixed with
ten times its weight of transparent gel. Beyond being somewhat brighter than
before, with a more pronounced warmth, the overall shape of its spectral
curve is preserved. On the other hand, the Transparent Yellow Oxide changes
dramatically, rising steeply through the bands of Yellow, Orange, and Red
wavelengths to take on a very warm tone.

The accompanying graph (Graph 12) traces changes in Chroma when varying
amounts of gel are added. As one can see, Yellow Oxide remains relatively
flat throughout, increasing only slightly as more and more gel is added. No
matter how transparent you make it, Yellow Oxide remains a muted color with
moderate saturation. On the other hand, while Transparent Yellow Iron Oxide
starts appreciably lower in overall Chroma, it actually increases
dramatically in saturation as gel is added, eventually surpassing Yellow
Oxide at the 1:1 mark and continuing to rise even further. Paradoxically,
perhaps, the color grows in brilliance as it is extended with gel, providing
proof -- should one ever be needed -- that it is the better choice for
creating luminous glazes.

Nickel Azo Yellow (PY 150) / Green Gold (PY 150, PG 36, PY 3)

These two colors, one a single pigment and the other a mix, present cases
where there are amazing changes in color when they are used to make a tint
or glaze. Green Gold, for example, starts its life as a darker, lower chroma
lime green in the masstone, but scrap it against a white surface or extend
it with Gel to reveal the undertone, and one quickly is confronted with the
much higher chroma of a bright yellow-green that is not easily created by
any other means. In the spectral curves where this is charted, one can see
the initial low-lying dark line that represents the color at full strength.
As increasing amounts of Regular Gel Gloss is added, there is a dramatic
upward swing in reflectance throughout the Green and Yellow ranges (500-
600nm) and a simultaneous almost complete absorption of Blues and Violets
that persists even at the extreme dilution of 1 part Green Gold to 50 parts
Regular Gel. It is this total suppression of the cooler bands, and the
peaking around 560nm, which is the region where the eye is most sensitive to
luminosity, that helps provide the striking vividness of this color.

Lastly, Nickel Azo Yellow (one of the component colors in Green Gold) does a
very similar dance, moving from a brownish, Raw Sienna-like hue at full
strength, then taking on increasingly translucent orange and yellowish earth
tones as more Gel is added, until finally reaching high-pitched yellow notes
reminiscent of a bright, transparent Hansa Yellow Medium. In the
accompanying graph, we track this movement in chroma when mixed with Gel as
well as with Titanium White. It is interesting to note that Titanium White
initially increases the chroma of Nickel Azo Yellow all the way through a
1:1 addition until finally succumbing to the neutralizing affect that
Titanium White will eventually have when added to any color.